DS16 prefers to take a thermos of hot food but that’s not helpful for you.
In summer he takes pasta or cous cous salad. Sometimes cold crumbed chicken. When he was younger his favourite was ‘nachos’ so a bento box with corn chips, salsa, guac, cheese, pico de gallo all in the separate compartments and then he’d put it all together. He doesn’t have time for that any more apparently.
Tortilla wraps are also good and you can load them up with fillings. He likes chicken, ham, smoked salmon with salad, avocado (mash it with lemon juice so it doesn’t go brown). DS also loves sour cream and will have it on anything- that would get the calories up! Chicken, lettuce, avo with sour cream and sweet chilli sauce is a favourite.
In addition to the above DS always has some fruit like an apple or mandarin/satsuma. Bag of crisps, homemade cookies if we have any. And always a couple of muesli bars chucked in for days when he’s extra hungry.
Secondary schools don’t have nut bans because by that age kids are learning to manage their allergies. DS isn’t a big fan of nuts but DD18 will often take some almonds or cashews.